Global navigational satellite systems (GNSS) include the global positioning system (GPS) and the global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are often utilized by military and civilian naval, ground, and airborne vehicles or platforms for navigation, targeting, and positioning applications.
In a GPS navigational system, GPS receiver units receive satellite or coded GPS signals from a set of twenty-four (24) Navstar satellites deployed in 12-hour orbits about the earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles in half geosynchronous orbits. The position of the GPS satellites is controlled and monitored by the Department of Defense (DoD). GPS satellites continuously emit coded GPS signals.
The GPS signal contains timing information that allows a user to determine the time elapsed for the GPS signal to traverse the distance between the GPS satellite and the user (the platform). By knowing the time the GPS signal left the GPS satellite, the time the GPS signal arrived at the user, and the speed of the GPS signal, the user can determine the distance from itself to the GPS satellite. By knowing the position of the GPS satellite (ephemeris data), and the distance from itself to the GPS satellite, the user can successfully triangulate its own position.
The GPS signal emitted by the satellites contains L-band carrier components at the transmitted frequencies of 1.575 GHz (L1) and 1.2276 GHz(L2). The L1 carrier component is phase shift keyed (PSK) modulated by two orthogonal pseudo-random noise (PRN) codes, a precise P(Y) code at a chipping rate of 10.23 MHz and a course acquisition (C/A) PRN code at a chipping rate of 1.023 MHz. Navigation data at 50 bits per second is modulo-2 added to each ranging code. The PRN ranging codes provide timing information for determining when the GPS signal was broadcast. The data component provides information, such as, the satellite orbital position. The L2 carrier is similar to the L1 carrier except that it contains either one but not both simultaneously PSK modulated P(Y) and C/A codes.
Position determination using a conventional GPS receiver is well known in the art. In conventional GPS, a receiver makes ranging measurements between an antenna coupled to the receiver and each of at least four GPS satellites in view. The receiver makes these measurements from the timing information and the satellite orbital position information obtained from the PRN code and data components of each GPS signal received. By receiving four different GPS signals, the receiver can make accurate position determinations.
The receiver acquires the satellite signals after down conversion by a direct injection local oscillator (LO). The LO is referenced and locked to a high quality crystal oscillator. The downconverted signal is quantized and digitally processed to determine PRN code position and the data component, hence, to calculate position information.
In prior art positioning system receivers, such as, a GPS receiver, a mechanical shock or sudden temperature transient can adversely affect the positioning determinations of the receivers. Generally, the mechanical shock or temperature transient can cause the GPS receiver to lose track of the satellite signals or can prevent the GPS receiver from acquiring the satellite signals.
More particularly, an external event, such as, a mechanical shock or a temperature transient, can degrade the accurate operation of the reference oscillator. The external event shifts the frequency of the signal generated by the reference oscillator, which causes the receiver to lose lock on the satellite signal if the receiver is tracking the satellite signal. If the receiver is not tracking the satellite signal, the frequency shift extends the amount of time required to acquire the satellite signal. The frequency shift results in these problems because the digital signal processing algorithms expect only nominal frequency shifts due to host vehicle or satellite motion.
External events, such as, sudden temperature transients and mechanical shocks, can occur in various stationary and mobile applications of GPS receivers. The delay in satellite acquisition time is particularly disadvantageous in GPS receivers utilized in gun-fired munitions or projectiles (e.g., GPS-guided ordinance). Generally, munitions can undergo an acceleration (e.g., a mechanical shock) of 8,000-30,000 G (One G=32.2 ft/sec.sup.2) of launch shock when fired. Any delay in initially acquiring the satellite can seriously adversely affect the ability of the receiver to provide positioning information because munitions traveling times are often very short. Reference oscillator frequency shifts of greater than +/-2 parts per million (ppm) can result from launch shocks.
In conventional systems, a sudden frequency shift due to an external event is indistinguishable from a Doppler shift caused by movement of the satellites or the GPS receiver. In response to a Doppler shift, the GPS receiver adjusts its local oscillator to center on the Doppler offset. It then scans (searches) the PRN ranging code in a window centered about the Doppler offset frequency to acquire or re-acquire the signal. The search is normally divided into several windows; each window representing approximately +/-0.24 ppm of the reference oscillator frequency shift. The receiver must scan all code positions in each window until the signal is acquired. The amount of time required to reacquire the signal can be great if a large number of windows must be searched.
In other applications, such as, aircraft, missiles, or ground or naval vehicles, temperature shifts and mechanical shocks can occur during normal operation. As stated above, in applications, such as, artillery launched GPS-guided projectiles, reference oscillator frequency shifts greater than +/-2 ppm can result from the 8,000-30,000 G launch shock. Further, tactical GPS-guided missiles can experience several hundred to several thousand G launch shocks, as well as major pyrotechnical shocks. A large frequency shift (e.g., +/-2 ppm) can require over eight frequency (Doppler) windows to be searched by the receiver post launch, thus extending satellite acquisition times.
Thus, there is a need for a positioning receiver that is less susceptible to losing signal lock due to mechanical shock or temperature transients. There is also a need for a positioning receiver that has a reduced delay due to the mechanical shock or temperature transients when initially acquiring the satellite signal. Further still, there is a need for a GPS receiver that can compensate for frequency changes due to mechanical shock.